1. Field of the Invention
This invention relates to communication systems, and more particularly to a method and apparatus for efficiently using bandwidth for subscriber unit initialization and synchronization in a time-synchronized communication system.
2. Description of Related Art
Time-synchronized communication systems are essential in modern society. Time-synchronized communication systems typically comprise sets of subscriber units or stations that communicate with one another. The communication system is “time synchronized” because a set of subscriber units is typically synchronized to a single time reference. Examples of time-synchronized communication systems include wireless communication systems and cable modem systems. As described in the commonly assigned related U.S. Pat. No. 6,016,311, wireless communication systems facilitate two-way communication between a plurality of subscriber radio stations or subscriber units (fixed and portable) and a fixed network infrastructure. Exemplary wireless communication systems include broadband wireless, satellite communication, mobile cellular telephone systems, personal communication systems (PCS), and cordless telephones. The key objective of these wireless communication systems is to provide communication channels on demand between the plurality of subscriber units and their respective base stations in order to connect a subscriber unit user with the fixed network infrastructure (usually a wire-line system). In the wireless systems having multiple access schemes, a time “frame” is used as the basic information transmission unit. Each frame is sub-divided into a plurality of “time slots”. Some time slots are used for control purposes and some for information transfer. Subscriber units typically communicate with a selected base station using a “duplexing” scheme thus allowing for the exchange of information in both directions of connection.
Transmissions from the base station to the subscriber unit are commonly referred to as “downlink” transmissions. Transmissions from the subscriber unit to the base station are commonly referred to as “uplink” transmissions. Downlink and uplink transmissions comprise “bursts” that are defined herein as data packets utilized for transmitting information between the base stations and the subscriber units. The base station maps and allocates bandwidth for both the uplink and downlink communication links. These maps are developed and maintained by the base station and are referred to as the Uplink Sub-frame Maps and Downlink Sub-frame Maps.
Propagation delays (i.e., time delays in transmissions between a transmitting unit and a receiving unit due to the distance or range between the units) occur within most communication systems. In time-synchronized communication systems, propagation delays must be determined because subscriber units are time synchronized to their respective base stations' time reference. Because a base station typically communicates with a plurality of subscriber units, the base station assigns to each subscriber unit unique time frames for receiving transmissions from the subscriber unit. Thus, a subscriber unit must transmit a burst to its associated base station during a particular designated time frame. For a burst to arrive from the subscriber unit to the base station “on time” (i.e., upon the occurrence of its designated time frame) the particular time of transmission should take into account propagation delays.
One example of time-synchronized communication is now described. In a wireless communication system, bursts travel through the atmosphere at approximately the speed of light (i.e., 3*108 m/s). If the range between a subscriber unit and its associated base station is 5 km, the propagation delay is 16.67 microseconds (3.33 microseconds/km*5 km). Thus, a base station sending a message to a subscriber unit has a propagation delay of 16.67 microseconds. The subscriber unit's response to the base station has another associated propagation delay of 16.67 microseconds. Thus, the round-trip propagation delay (i.e., total delay for a burst to travel from the base station to the subscriber unit and for the subscriber unit to respond to the burst by sending a message to the base station) is approximately 33.3 microseconds (16.67+16.67). Round-trip delay is also commonly referred to as “Tx time advance”. For a subscriber unit to be time-synchronized to the base station's time reference, the subscriber unit therefore must transmit its burst 33.3 microseconds early. Time-synchronization between a subscriber unit and a base station consequently depends upon knowledge of the round-trip delay or range between the subscriber unit and the base station.
Disadvantageously, problems occur during initialization processes between the base station and the subscriber units. Problems occur when a subscriber unit initially accesses the base station because the subscriber unit's round-trip delay (or range) is initially unknown. If the round-trip delay is unknown, a burst can arrive at a time frame assigned to a different subscriber unit and thereby cause “collisions” (i.e., bursts from different subscriber units arrive at the base station simultaneously). Collisions can degrade a communication system's performance because a base station can typically receive transmissions (i.e., bursts) from only one subscriber unit at any given moment in time. Thus, a mechanism for providing initialization and synchronization between a plurality of subscriber units and their associated base station is needed.
One method for providing initialization and synchronization between a plurality of subscriber units and base stations is known as the “Random Access Burst” (RAB) method and is described in detail in a book by Siegmund M. Redl, Matthias K. Weber and Malcolm W. Oliphant; entitled “An Introduction to GSM” appearing at section 5.8.2 (pages 84, 85 and 95), published in 1995, and hereby incorporated by reference herein for its teachings on initialization and synchronization procedures in wireless communication systems. The RAB method described by Redl et. al. takes advantage of “timing opportunities” (periods of time assigned for subscriber unit initialization and synchronization purposes) during which subscriber units that have not resolved their round-trip delay or Tx time advance (i.e., not yet synchronized with the base station's time reference) may transmit without interfering with other subscriber units that have already resolved their round-trip delay or Tx time advance (i.e., subscriber units that have already synchronized with the base station's time reference). In the RAB method, a subscriber unit utilizes a “random access burst” when initially attempting to communicate with its associated base station.
FIG. 1 shows the structure of a random access burst in accordance with the Random Access Burst method. The random access burst comprises message bits (m) 2 and guard bits (g) 4. The message bits 2 contain information regarding synchronization and identification of the subscriber unit. The length of the message bits 2 determines a time period known as the “m” time period because each bit requires a certain length of time to transmit. The guard bits 4 provide a mechanism for preventing collisions. The length of the guard bits 4 determines a time period known as the “g” time period. The g time period represents the maximum round trip delay possible in a communication system (i.e., a situation where the subscriber unit is at a maximum distance from the base station as determined by the base station's capabilities). For example, in a wireless communication system wherein the maximum distance from the subscriber to the base station is 37.75 km, the maximum round trip distance is 75.5 km (2*37.75). Thus, the maximum round trip delay is approximately 252 microseconds (75.5 km*3.33 μs/km). In the example, the length of the guard bits 4 must be a minimum of 68.25 bits because each guard bit requires 3.69 microseconds to transmit (i.e., 252 μs/3.69 μs/bit=68.25 bits).
The RAB method reserves various time frames in the uplink called “timing opportunities” for subscriber units that have not resolved their round-trip delay or Tx time advance (i.e., subscriber units that have not yet synchronized with the base station's time reference). A timing opportunity must be sufficient in duration to accommodate subscriber units that are at the maximum range of the base station. Thus, referring to FIG. 1, the duration of the timing opportunity must be equal to at least the time period represented by the random access burst (i.e., m time period+g time period) in order to accommodate a subscriber unit that is at a maximum range from the base station.
FIG. 2 shows the time sequence of a random access burst arriving at a timing opportunity in accordance with the Random Access Burst method. In FIG. 2, a timing opportunity exists at a time frame n of an Uplink Sub-frame (as described in more detail below with reference to FIG. 3). As shown in FIG. 2, the timing opportunity begins at an instant in time known as a “Start of Opportunity” time instant and ends at an instant in time known as an “End of Opportunity” time. In accordance with the RAB method subscriber units begin transmitting a random access burst at the Start of Opportunity time (i.e., at the beginning of a timing opportunity). As shown in FIG. 2, the message bits 2 arrive at the base station at a later time known as an “Arrival of Message” time instant. The time period between the Start of Opportunity time instant and the Arrival of Message time is known as a “Ttwoway” period of the random access burst. The base station can calculate a round-trip delay because the Ttwoway period's time duration is equal to the round-trip delay's time duration. The time period it takes for a burst to transmit between the base station and a subscriber unit is known as the “Toneway” period of the random access burst. The Toneway period is exactly one-half of the Ttwoway period. The message bits 2 transmission terminate at a time known as an “End of Message” time instant. The time period between the End of Message time and the End of Opportunity time instant is known as an “Unused Time” period because no information is received during this time period.
The g time period (FIG. 1) is equal to the sum of the Unused Time period and the Ttwoway time period. The Unused Time period is required in order to accommodate the possibility of a maximum round trip delay. Only subscriber units that are at a maximum distance away from the base station have Unused Time periods of zero microseconds. As most subscriber units are within the maximum distance from the base station, Unused Time periods are typically greater than zero microseconds.
In accordance with the RAB method, only one subscriber unit can synchronize with the base station's time reference during a time opportunity. FIG. 3 shows an exemplary Uplink Sub-frame Map of the Random Access Burst method. As shown in FIG. 3, the RAB method schedules timing opportunities (R) 8 at separate and distinct time frames. The exemplary Uplink Sub-frame Map of FIG. 3 comprises 51 time frames consecutively numbered from 0 to 50. As described above with reference to FIG. 1, each timing opportunity 8 must be sufficient in duration to accommodate subscriber units that are at the maximum range of the base station.
Disadvantageously, the RAB method inefficiently allocates bandwidth because a base station desiring to receive x subscriber units that have not resolved their Tx time advance, where x is an integer, must allocate at least x(m+g) total time to minimize burst collisions. Burst collisions may occur because timing opportunities are typically directed to more than one subscriber unit. For example, a base station desiring to receive 5 subscriber units must allocate at least 5(m+g) total time to minimize burst collisions. As shown in FIG. 2, subscriber units that are within the maximum distance from the base station have an Unused Time period greater than zero microseconds. The Unused Time period decreases the overall bandwidth of a communication system because during this unused time period data is neither transmitted nor received. Therefore, bandwidth allocation using the RAB method is inefficient and the RAB method therefore disadvantageously suffers from a decrease in overall bandwidth availability.
Therefore, a need exists for a method and apparatus for efficiently using bandwidth for initial communication and synchronization in a time-synchronized communication system. The method and apparatus should decrease the amount of bandwidth that a communication system requires for initial synchronization purposes, thereby increasing the overall bandwidth availability. Such method and apparatus should be efficient in terms of the amount of bandwidth consumed by the initial synchronization message that is exchanged between the plurality of subscriber units and their associated base stations. The present invention provides such an initial communication and synchronization method and apparatus.